1.Field of the invention
The present invention concerns the manufacture of a buried stripe semiconductor laser. The semiconductor structure of a laser of this kind is known internationally as a "planar buried heterojunction" (PBH) structure.
2.Description of the prior art
One prior art method of manufacturing a PBH laser includes so-called layer growing steps during which the semiconductor layer thickness is increased by epitaxial deposition of the material of the layers. This process is briefly as follows (see FIG. 1):
Growth of binary composition, then quaternary composition and then binary composition semiconductor layers on a substrate. This produces a double heterostructure.
Growth of a ternary composition contact layer having a forbidden band energy that is lower than that of the previous binary composition layer.
Definition of a laser stripe in the quaternary composition layer by chemical etching from the top surface of the substrate using a dielectric mask.
Growth of two lateral semiconductor layers to surround and bury the laser stripe.
Localized deposition of a contact layer through a window formed on top of the laser stripe in a dielectric layer previously deposited for this purpose onto the top surface of the substrate.
The lateral layer growth step conventionally employs so-called "selective" growth methods which have various drawbacks including a limited yield. However, they enable localized growth of layers on the semiconductor material with no deposit of material on top of the laser stripe. This requires retaining the mask which was used to define the laser stripe during this growth step. This growth step is carried out at temperatures above 500.degree. C. For this reason the dielectric mask used is formed by depositing and etching a dielectric layer.
This prior art process is described, for example, in an article by M. HIRAO et al "Long wavelength InGaAsP/InP buried heterostructure laser", J. Opt commun., 1980 , 1, pp. 10-14.
Laser semiconductors are known in some cases to show deterioration of their properties with time and this may be the result of pollution during manufacture.
Also, forming a contact localizing window to enable localized injection of current into the laser stripe requires at least one photo-etching stage requiring delicate realignment.
An NTT document, PATENT ABSTRACTS OF JAPAN vol.12, No.449(E-686) (3296) Nov. 25, 1988,abstract of document JP-A-63 177 493(NTT) July 21, 1988, describes the manufacture of a planar buried heterojunction laser in which lateral layers are formed by a non-selective growth method which grows the semiconductor material not only at the sides of the laser stripe but also on top of it to create a parasitic projection. This is subsequently removed. No arrangement is described for localizing the electrical contact on top of the laser stripe. If injection of current into the laser stripe is not localized, the performance of the laser is seriously degraded.
The objects of the present invention include:
simplifying a method of manufacturing a planar buried heterojunction semiconductor laser,
enabling the use of high-yield growth processes within this method,
reducing the risk of pollution during formation of the semiconductor structure of the component,
avoiding realignment problems associated with contact localization on the component, and
generally, enabling the use in the method of processes which may be advantageous as compared with processes previously employed.